Glass syringes are considered by medical practitioners to be the most sensitive of all the types of syringes currently produced. This extreme sensitivity is a result of the precise tolerances utilized in the manufacturing process. Typically, in these manufacturing procedures, both the exterior surface of the syringe plunger (piston) and the interior surface of the barrel in which it is to be seated for administering fluid are treated to a grinding and/or bead-blasting step which imparts a very fine texture to these mating surfaces. This process when combined with a very close tolerance fit results in a closely matched assembly which does not exhibit the typical drag or frictional interference associated with syringes manufactured from plastic or other non-glass materials as the plunger moves axially towards the distal (leading end) of the barrel.
The matched ground glass syringes provide a smooth, friction-free movement of the syringe plunger within the syringe barrel effective in the movement of either air or liquid through the syringe. Although the plunger and barrel surfaces do not actually touch, assuming the plunger moves forward axially in the barrel, their close proximity entraps air or liquid molecules between their surfaces sufficient to create an air- or liquid-tight interface. This close fit results in an effortless seal between the plunger and barrel. Total expulsion of air or liquid from the syringe is accomplished without any leakage from between the mating surfaces and is further accomplished with a minimal and, in some cases, immeasurable amount of force on the plunger.
Although the attributes of the matched component ground glass syringes are uncontested, they are not without certain limitations or inherent problems.
As was mentioned above, the plunger and barrel surfaces do not actually touch provided that the plunger moves forward axially within the barrel.
However, as will be discussed in detail hereinafter, there is a tendency in the manipulation of the plunger for it to move distally in the barrel in a non-axial manner which, in turn, will cause "sticking" and "freezing up" such that the plunger becomes inoperable for its intended use. This problem is repeatedly described in the medical literature, including, but not limited to training texts for epidural anesthesia, and the patent literature, such as Col. 1, lines 10-29 of Applicant's U.S. Pat. No. 5,397,313 for a LOW FRICTION SYRINGE issued Mar. 14, 1995.
If this problem occurs during the precise procedure described as the "loss of resistance technique" for locating the epidural space, the anesthesiologist will be thwarted in the attempt to locate the epidural space and the patient is put into certain risk in the administration of epidural anesthesia.
The technique for locating the epidural space is well described in the literature (citations not needed) describing the loss of resistance technique. As described and well known by those skilled in the art, a glass syringe is filled with air, liquid or a combination of both and attached to an epidural needle. Once so attached, the needle is advanced through the delicate tissues of the spineous ligaments of the patient. Constant or pulsating pressure is applied to the syringe plunger while the tip of the epidural needle moves forward through the interspineous tissues. A tactile sensation of resistance to the forward movement of the plunger and the spring-like rebound of the plunger when pressure is released indicate to the anesthesiologist that the needle is still within the interspineous ligaments and that the precise location of the epidural space has not been reached.
If the plunger should suddenly stick or freeze up, then the entire procedure is put at risk because the needle cannot, with certainty, be advanced toward the epidural space. Furthermore, location of the epidural space will not be indicated by the free movement of the syringe plunger. When the plunger sticks or freezes up, then the procedure must be abandoned until the syringe is replaced.
It is apparent from the aforementioned description that limitations of a significant nature exist with the current, state of the art, glass syringes. It is further apparent that there exists a need for an improved glass syringe.
Stated simply, the task of the present invention is to provide a glass syringe which does not exhibit the foregoing limitations which may be described as sticking or freezing up.
These limitations occur most frequently when pressure applied to the plunger moves the plunger out of axial alignment with the axis of the syringe barrel. This misalignment of plunger and barrel results in the leading edge and the surface of a trailing portion of the plunger binding or dragging against the ground glass surface of the bore of the syringe barrel. The cushion of air or liquid which imparts the friction-free movement of the plunger through the barrel is eliminated and the ground glass surfaces drag on one another or bind together, resulting in the limitations or deficiencies inherent in the system known in the art as sticking or freezing up.
Due to the low frictional resistance between the glass plunger and glass barrel, a high degree of sensitivity to pressure changes is imparted to the fingers of the anesthesiologist. This sensitivity is of paramount importance to the success of the procedure and, ultimately, to the correct placement of the epidural needle within the epidural space. The sensitivity so described is dependent upon the anesthesiologist's manipulation of the syringe plunger and herein lies the basis and cause of the aforementioned limitations to which the task of this invention is directed to obviating.
It is pointed out that the human hand is not completely capable of holding a syringe barrel between the index and middle fingers and simultaneously to pulsate the syringe plunger without imparting a lateral or non-axial force to the plunger. This mis-aligning motion was readily apparent to Applicant after he studied and analyzed the motion of the thumb in relation to the adjoining index and middle fingers; and Applicant believes it will be equally obvious to those skilled in the art in the light of the foregoing description.
Specifically, the thumb describes an arc as it pivots from the thumb joint in relation to the stationary first and second fingers. This unaligned motion imparts a lateral force to the plunger as it is moved within the barrel.
The following patents have been found as the result of a novelty search of the subject matter of this application. No other prior art relevant to the present invention is known to Applicant or his attorney.
U.S. Pat. No. 782,723 issued to Campbell relates to a hypodermic syringe for antitoxin serum having for its object the provision of an improved piston and operating handle therefor by the use of which it is possible to impart a rotary motion to eject serum from the barrel of the syringe, even though the piston adheres firmly to the walls of the barrel, due to the use of cement or by the gumming action of the serum itself. The patent discloses a glass barrel and a rubber piston which can be actuated with a screwdriver-like projection E (FIG. 4). It appears that the screwdriver-like projection is moveable within a slot .function. disclosed in FIG. 4. Although it is conceivable that the screwdriver-like projection could be rotated within the slot, such rotation is not disclosed.
U.S. Pat. No. 2,102,591 to Hagemeier relates to a dental syringe of the kind wherein the plunger or piston is operated by a pair of hand grips, the object of the invention being to obtain a syringe of this kind which allows of a gradual, stepwise discharge of its contents in predetermined quantities and in which the plunger is in the form of a screw spindle on which the operating elements act through the medium of an adjustable nut by which the position of the syringe can be regulated. In Col. 2, page 1, in the discussion of using the syringe for filling teeth with cement, it is disclosed that a piston 26 may be employed to which the plunger 17 may be connected by means of a ball joint, the ball 27 being attached or formed with a socket 28 applied to the plunger.
U.S. Pat. No. 2,354,649 to Bruckner also relates to a dental syringe which has a novel form of plunger designed to prevent the disruptive wedging forces occurring with axial misalignment of the plunger and barrel of a syringe. The syringe accommodates insertable glass vials having a rubber plug closure at each end. The discharge end of the syringe is provided with a hollow needle which punctures one of the plugs, while the other end of the syringe is provided with a finger grip and plunger, the plunger being axially slidable and engageable with the other of the plugs, the other plug then acting as a piston to eject fluid from the vial. An object of the invention (col. 1) is stated to be to permit self-adjustment of the plunger head with respect to the vial and piston and thus to eliminate "those side thrusts or strains on the vial walls, due to this wedging action" . Another object is to provide a plunger made in two parts and having a swivel connection between the parts. This is accomplished with a ball and joint linkage. "Since the plunger head is free to swivel slightly relative to the piston, there results a three link kinematic chain, comprising the piston and two parts of the plunger, in contrast with the self jamming of the two link trains of the prior art."
U.S. Pat. No. 2,832,340 issued to Dann et al. relates to a "syringe push rod" attached to a plunger and being especially adapted for use in disposable syringes. When the syringe is filled with fluid, the push rod has a relatively short bearing on the interior surface of the syringe and it is said that the push rod may inadvertently be thrown out of axial alignment by the user. With prior push rods rigidly attached to the plunger, it is said that cocking of the plunger will occur and the cocked plunger will allow a small amount of drug seepage proximally. This problem is said to be obviated by providing a flexible connection between the push rod and the plunger. As disclosed, means are provided at the distal end of the push rod for positive engagement with the syringe plunger providing positive control of aspiration and injection, the means being attached to the push rod by a flexible web or elastic diaphragm, thus permitting a limited degree of axial misalignment of the push rod without a corresponding cocking of the plunger in the syringe barrel. The means for connecting the push rod to the plunger may, for example, be a socket mating with a threaded stud on the plunger, or a stud mating with a threaded metal socket in the plunger, or a threaded stud mating with a threaded recess molded in the plunger material.
U.S. Pat. No. 3,175,646 granted to Wilcox relates to a structure for connecting a piston and a piston rod adapted to permit a degree of universal movement therebetween to provide a valve action controlling the passage of fluid through the piston, e.g. for a damping dashpot for a sensitive electromagnetic assembly. The stated task is stated to be either or both of the following: (1) permit the piston rod a limited degree of universal movement without placing any strain on the piston itself; and (2) provide a structurally simple connection between the piston rod and piston which will provide for the transmission of movement from one to the other and which at the same time function as a directional valve. With reference to claim 1, the invention is a piston assembly comprising "a member oppressively connected to said piston [at a given end] . . . for angular movement relative thereto", the improvement of the assembly being "said piston assembly including an element having a chamber opening onto said given end of said piston, said member extending into said chamber and having a ball-like part ratably received therein with air-flow clearance between itself and said chamber." The intent of the patent is to allow the transfer of fluid between the fore and aft areas of the cylinder as separated by the piston which permits air-flow therebetween. The universal or angular movement of the member and piston is incidental to the accomplishment of air-flow through the piston, the described "improvement" . (As will be more apparent hereinafter in the discussion of the instant invention, the intent to allow the transfer of fluid is different from the present invention whose objective is to accomplish total fluid separation between piston and barrel areas of the syringe.
Finally, for the sake of full candor, Italian Patent No. 708,474 to Nogier et al. was also found. Applicant can review the patent drawings but is unable to read the text. Although it appears from FIG. 3 that the plunger 1 may be flexibly attached at neck 8 to the piston 7, it appears that the piston is made of flexible rubber rather than glass.